Method for mnaging particulate filter backwashing means

ABSTRACT

A method is provided for backwashing, by heating, a particulate filter to be purified of exhaust gases (G) of an internal combustion engine (M), in particular an internal combustion engine equipping a motor vehicle (V). The method includes controlling the start-up of a device for heating ( 1 ) the filter (F) when the engine (M) delivers an engine torque at a predetermined low threshold (C bas ).

The present invention relates to a method of backwashing, by heating, aparticulate filter adapted to purify exhaust gases from an internalcombustion engine, in particular an internal combustion engine equippingan automobile vehicle.

A particulate filter conventionally comprises honeycomb porousstructures forming filter bodies for filtering particles emitted bydiesel vehicles. These filter bodies are generally made of ceramics(cordierite, silicon carbide, etc.). They may be of one-piececonstruction or consist of various units. In the latter case, the unitsare assembled by bonding them together with a ceramic cement. Theresulting assembly is then machined to the required cross section, whichis generally round or oval. The resulting filter body may comprise aplurality of passages, closed at one end or the other, which may havecross sections with different shapes and diameters. The filter body isgenerally inserted into a metal enclosure, for example as described inFR-A-2 789 327.

In use, soot accumulates in the passages of the filter body, inparticular in the inlet passages, which increases the head loss causedby the filter body and therefore degrades the performance of the engine.For this reason, the filter bodies must be regenerated regularly, forexample every 500 kilometers.

Regeneration, also known as “backwashing”, consists in oxidizing thesoot. It is necessary to heat the filter to achieve this. The flashpoint of soot is of the order of 600° C. under standard operatingconditions but the temperature of the exhaust gases is on average onlyof the order of 300° C. However, it is possible to add additives to thefuel to catalyze the soot oxidation reaction and reduce the flash pointto approximately 150° C.

The exhaust gases or the filter body may be heated, or the soot may beheated directly. Different techniques have been developed.

One recent approach is local heating, on the upstream side of the filterbody, in such a manner as to initiate combustion, which then propagatesprogressively into the whole of the filter body. This type of techniqueis described in FR A 2 771 449 and DE-A-19530749, for example.

The heating means are generally connected to an electrical power supplyof the vehicle.

They may comprise ceramic plugs, for example diesel engine glow plugs,simple electrical elements, or ceramic igniters, as described in Frenchpatent application No. 0013998 filed Oct. 31, 2000 by the Applicant.

The engine operating conditions may vary considerably between startingthe regeneration process, i.e. turning on the heating means, for examplethe ceramic igniters, and the moment at which the soot actually ignites.Variation in operating conditions can greatly degrade efficiency.

The object of the invention is to provide a backwashing method thatlimits the degradation of the efficiency of regeneration of the filter.

The above object is achieved by a method of backwashing by heating aparticulate filter for purifying exhaust gases from an internalcombustion engine, in particular an internal combustion engine equippingan automobile vehicle, noteworthy in that starting of the heating meansfor the filter is commanded when said engine is delivering an enginetorque below a predetermined bottom threshold.

It has been found that, under these engine operating conditions theignition of the soot accumulated in the filter body of the filter andthe propagation of the combustion of the soot are not greatly influencedby the engine operating conditions. Thus by controlling the heatingmeans in accordance with the invention, efficient regeneration of thefilter is achieved, substantially independently of variations in theengine operating conditions.

Thus the starting of said heating means is preferably commanded only ifthe engine is delivering an engine torque below said low threshold.

According to other features of the invention:

-   -   starting of said heating means is commanded when said engine is        idling or in the absence of combustion in said engine;    -   said bottom threshold is less than or equal to 10% of the        maximum engine torque of said engine;    -   starting of said heating means is commanded after said engine        has delivered an engine torque exceeding a top threshold for at        least a predetermined time period;    -   said top threshold is greater than or equal to 30% of the        maximum engine torque of said engine;    -   said predetermined time period is greater than or equal to one        minute;    -   starting of said heating means is commanded only if said filter        and/or said engine are at temperatures exceeding respective        threshold temperatures;    -   starting of said heating means is commanded after detection of a        minimum mass of soot in said filter;    -   starting of said heating means is commanded at the latest one        minute after said engine has begun to deliver an engine torque        below said bottom threshold;    -   starting of said heating means is commanded as soon as said        engine has begun to deliver an engine torque below said bottom        threshold;    -   said heating means are maintained in operation for at least five        seconds after starting;    -   said engine torque delivered by said engine is evaluated by        measuring a position of an accelerator pedal;    -   said heating means comprise a hot tip whose temperature        increases from a temperature greater than or equal to 20° C. to        a temperature greater than or equal to 1000° C. within six        seconds of being started;    -   said heating means comprise Mini-Igniter® type igniters.

The invention also provides a device for implementing the abovebackwashing method, said device comprising heating means for heatingsaid filter, control means for controlling said heating means, and acomputer for managing said control means.

The device is noteworthy in that it further comprises detection meansfor detecting the delivery by said engine of an engine torque below apredetermined bottom threshold, said computer commanding starting ofsaid heating means in response to said detection.

According to other features of the device of the invention:

-   -   said heating means are adapted to heat by direct contact        particles deposited on a filter body of said filter;    -   said heating means comprise at least one ceramic igniter;    -   hot tips of said heating means are disposed inside said filter        body or on an outside lateral surface of said filter body,        preferably without projecting from said surface;    -   the device includes detection means for detecting rotation of        said engine.

Other features and advantages of the present invention will becomeapparent on reading the following description and from the appendeddrawing in which the single figure is a diagrammatic representation of adevice constituting a preferred embodiment of the invention.

The device of the invention represented in FIG. 1 comprises ceramicigniters 1 disposed in such a manner as to be able to heat a particulatefilter F for purifying exhaust gases G from an internal combustionengine M of a vehicle V.

The ceramic igniters may equally be disposed in such a manner as to heatthe gases G or to heat the soot directly.

The ceramic igniters 1 are small components which, when an electricalcurrent flows in them, are locally heated to a very high temperature(1200 to 1400° C.). These igniters are usually made of a highlyresistive ceramic material such as silicon carbide, sometimes mixed withother ceramic components.

The ceramic igniters are preferably those described in U.S. Pat. Nos.5,085,804 and 5,045,237.

The igniters described in U.S. Pat. No. 5,085,804 comprise a hot regionconsisting primarily of a sintered mixture of 5 to 50% by volume ofmolybdenum disilicide and 50 to 95% by volume of a mixture of siliconcarbide and silicon nitride.

The open pore content is less than or equal to 4% and the bendingresistance at the standard threshold is at least 207 MPa (30 000 psi).

The ratio between the resistivity at room temperature and theresistivity at 1200° C. of the igniters described in U.S. Pat. No.5,085,804 is less than 19.8, but may be as low as 0.2. A ratio of thismagnitude indicates a very short response time, the response timevarying in the same direction as this ratio.

The hot region of the igniters described in U.S. Pat. No. 5,085,804 maybe defined as follows:

(1) Thickness or width of at least 0.0508 cm (0.020 in) or cross sectionarea of at least 0.00258 cm² (0.0004 in²).

(2) Thickness or width of at most 0.127 cm (0.050 in) or cross sectionarea of at most 0.0161 cm² (0.0025 in²).

(3) For narrow cross sections, hot region lengths at most approximately2.53 cm (1 in), the length/area ratio being at most approximately 2500to 0.0258 cm² (0.004 in²).

(4) For short, fat shapes, a hot region length of at least 0.508 cm (0.2in).

The igniters described in U.S. Pat. No. 5,045,237 have a hot regionconsisting primarily of a sintered mixture of 5 to 50% by volume ofmolybdenum disilicide and 50 to 95% by volume of a mixture of siliconcarbide and aluminum nitride. The other features and performance ofthese igniters are similar to those of the igniters described in U.S.Pat. No. 5,085,804.

Detailed information on the structure and the fabrication of the ceramicigniters may be found in NORTON COMPANY's U.S. Pat. Nos. 5,191,508,5,085,804, 5,045,237, 4,429,003 and 3,974,106.

The ceramic igniters 1 are preferably disposed to heat particlesdeposited on a filter body of the filter.

The hot tips of the ceramic igniters 1 are preferably inside the filterbody of the filter F, the ceramic igniters being pushed wholly orpartially into the filter body.

This has the advantage of enabling efficient transmission of heat energyto the soot accumulated in the filter body of the filter F.

In one variant of the invention, the ceramic igniters are on thesubstantially cylindrical outside lateral surface 2 of the filter body,and are preferably pushed into housings, which are preferably ofcomplementary shape, formed in the surface 2. The shape of the housingsis preferably determined so that the igniters do not project from thesurface 2.

For more detailed information on the disposition of the ceramic igniterssee French patent application No. 0013998.

However, the invention is not limited to heating through direct contactof the heating means with the particles of soot. The exhaust gases orthe filter body may be heated.

The ceramic igniters 1 are connected by electrical wires 3 to controlmeans 5 adapted to supply them selectively with an ignition electricalcurrent.

According to the invention, this device further comprises a computer 7for managing the control means 5 and evaluation means 9 for evaluatingthe engine torque delivered by the engine M, for example by measuringthe position of an accelerator pedal 10.

The evaluation means 9 for evaluating the engine torque are connected bya line 11 to the management computer 7.

The device of the invention preferably further comprises measuring means13 for measuring the clogging of the filter F and detection means 15 fordetecting rotation of the engine M transmitting data to the computer 7via lines 17 and 19, respectively.

The device of the invention operates in the following manner.

The measuring means 13 for measuring the clogging of the filter F informthe computer 7 if they detect a degree of clogging necessitating thestarting of a regeneration operation.

The computer 7 advantageously verifies that the following conditions a)and b) are satisfied before turning on the ceramic igniters 1:

-   a) The engine M is running, which enables renewal of the air in the    filter F. If the air is not renewed, the combustion of the soot    quickly stops.    -   The computer 7 is informed by the means 15 of the running or        stopped state of the engine M.-   b) The filter is warm, i.e. its temperature exceeds a minimum    temperature.    -   The computer 7 preferably considers that this latter condition        is satisfied when the engine M has just delivered an engine        torque exceeding a predetermined top threshold C_(top) for a        predetermined time period, for example while the vehicle V is        travelling a few kilometers.    -   The top threshold C_(top) is preferably set to a value greater        than or equal to 30% of the maximum engine torque C_(max) that        the engine M may provide.    -   The predetermined time period is preferably at least one minute.

According to the invention, when the two conditions a) and b) aresatisfied, the computer 7 waits for a third condition c) to besatisfied, namely that the engine M is delivering an engine torque belowa predetermined bottom threshold C_(bottom).

The bottom threshold C_(bottom) is preferably less than or equal to 10%of the maximum engine torque C_(max).

The engine M delivers an engine torque below the bottom thresholdC_(bottom) in two main situations in particular: in the absence ofcombustion, and when idling.

The term “absence of combustion” (in the engine) refers to phases ofoperation of the engine during which no combustion occurs in any of thecylinders of the engine.

In a vehicle propelled by an internal combustion engine, these phasesoccur in particular when the injection of fuel is cut off because thedriver of the vehicle ceases to depress the accelerator pedal 10. In ahybrid “thermal—electrical” vehicle, these phases may also occur whenthe electric motor is supplying all of the power for propelling thevehicle. For regenerating the filter F, it is nevertheless necessary tokeep the thermal engine M running, to satisfy condition a).

The evaluation means 9 supply to the computer 7 an estimate of the motortorque delivered by the motor M as a function of the position of theaccelerator pedal 10. This allows the computer 7 to determine when theengine M is delivering an engine torque below the bottom thresholdC_(bottom) or above the top threshold C_(top).

The condition c), whereby the engine M must deliver an engine torquebelow the bottom threshold C_(bottom), is required prior to turning onthe ceramic igniters in order for the exhaust gases G coming from theengine M and entering the filter F to retain a high oxygen content, toencourage ignition of the soot present in the filter F and thepropagation of combustion to all of the soot in the filter body.

As soon as possible after the condition c) is satisfied, and preferablywithin a maximum delay of one minute, the computer 7 causes the controlmeans 5 to pass an ignition electrical current through the ceramicigniters 1.

In one variant of the invention, the computer 7 commands the turning onof the ceramic igniters 1 as soon as condition c) is satisfied, withoutnecessarily checking the degree of clogging of the filter beforehand, orits temperature, or for rotation of the engine. This reduces theprobability of ignition of the soot, but simplifies the architecture ofthe device.

The ceramic igniters 1 have a very short response time. Althoughstandard spark plugs take from 10 to 40 seconds to reach 1000° C., theceramic igniters 1 take only 3 to 6 seconds to achieve the sametemperature. This is crucial since if heating is not fast enough, thesoot tends to be consumed without igniting, rather than tending toignite, which impedes the propagation of combustion.

The ceramic igniters 1 preferably have a rate of increase of temperatureexceeding 150° C./s, more preferably greater than 200° C./s, and evenmore preferably greater than 300° C./s.

The ceramic igniters preferred for the invention are Mini-Igniters® fromSAINT-GOBAIN ADVANCED CERAMICS, the characteristics of which aresummarized in Table 1 below. TABLE 1 Preferred Mini-Igniters ®Mini-Igniter ® model 300 401 601 405 600 Time for temperature 3 s 3 s 5s <2 s <6 s to increase to nominal temperature from room temperatureContinuous electrical 1.5 to 2.5 A 1.0 to 2.2 A 0.4 to 1.2 A 0.4 to 0.6A 0.2 to 0.75 A current at 12 V at 24 V at 120 V at 24 V at 120 VElectrical resistance 1.0 to 6.0 1.0 to 6.0 25 to 300 1 to 100 25 to 600at room temperature Ohms Ohms Ohms Ohms Ohms Nominal temperature from1035 1275 to 1275 to from 1150 to to 1580° C. 1455° C. 1455° C. 1050 to1400° C. at 24 V at 120 V 1500° C. at 120 V

Given the voltages usually available in automobile vehicles, the 300models operating at 12 volts and the 401 and 405 models operating at 24volts are preferred.

The phases of operation of the engine during which it delivers an enginetorque below the bottom threshold C_(bottom) generally last a fewseconds, for example the time taken by the vehicle V to slow down andstop. For these few seconds to be sufficient to turn on the heatingmeans and ignite the soot, it is advantageous for those means to have avery short response time.

The ceramic igniters 1 are therefore particularly adapted to themanagement method of the invention, especially as they provide forpositioning the hot tips within the filter body.

The computer 7 maintains the ignition electrical current through theceramic igniters 1 for a predetermined time period, preferably at least5 seconds.

The ignition electrical current is preferably maintained throughout thispredetermined time period, even if in the meantime the engine torquesupplied by the engine M again exceeds the bottom threshold C_(bottom).

The method of the invention is advantageously applicable to any heatingmeans, but especially heating means whose response time enables the sootto be ignited during a phase of absence of combustion, i.e. within amaximum time period of around 10 seconds in the case of a thermalvehicle.

The combustion of the soot continues until all of the soot has beenconsumed or the conditions for combustion are no longer satisfied. Inthe latter case, the soot is extinguished spontaneously.

Of course, the present invention is not limited to the embodimentdescribed and shown by way of illustrative and nonlimiting example.

In particular, it is not limited to one particular type of filter. Forexample, it is applicable whether the filter F is catalytically assistedor not.

It also encompasses filters through which pass exhaust gases resultingfrom the combustion of a fuel to which one or more additives have beenadded, in particular an additive intended to catalyze the reaction ofoxidation of the soot and/or to reduce its flashpoint.

Finally, any heating means having a hot tip whose temperature may beincreased from room temperature of 20° C. to a temperature of 1000° C.or greater within 6 seconds from its nominal energization is suitablefor implementing the invention.

1. Method of backwashing by heating a particulate filter (F) forpurifying exhaust gases (G) from an internal combustion engine (M), inparticular an internal combustion engine equipping an automobile vehicle(V), wherein starting of the heating means (1) of the filter (F) iscommanded when said engine (M) is delivering an engine torque below apredetermined bottom threshold (C bottom), said heating means comprisingat least one ceramic igniter adapted to heat, by direct contact,particles deposited on a filter body of said filter (F) and adapted toreach a temperature of 1000° C. within 3 to 6 seconds.
 2. Methodaccording to claim 1, wherein starting of said heating means (1) iscommanded when said engine (M) is idling.
 3. Method according to claim1, wherein starting of said heating means (1) is commanded in theabsence of combustion in said engine (M).
 4. Method according to claim1, wherein said bottom threshold (C_(bottom)) is less than or equal to10% of the maximum engine torque (C_(max)) of said engine (M).
 5. Methodaccording to claim 1, wherein starting of said heating means (1) iscommanded after said engine (M) has delivered an engine torque exceedinga top threshold (C_(top)) for at least a predetermined time period. 6.Method according to claim 5, wherein said top threshold (C_(top)) isgreater than or equal to 30% of the maximum engine torque (C_(max)) ofsaid engine (M).
 7. Method according to claim 5, wherein saidpredetermined time period is greater than or equal to one minute. 8.Method according to claim 1, wherein starting of said heating means (1)is commanded only if said filter (F) and/or said engine (M) are attemperatures exceeding respective threshold temperatures.
 9. Methodaccording to claim 1, wherein starting of said heating means (1) iscommanded after detection of a minimum mass of soot in said filter (F).10. Method according to claim 1, wherein starting of said heating means(1) is commanded at the latest one minute after said engine (M) hasbegun to deliver an engine torque below said bottom threshold(C_(bottom)).
 11. Method according to claim 1, wherein starting of saidheating means (1) is commanded as soon as said engine (M) has begun todeliver an engine torque below said bottom threshold (C_(bottom)) 12.Method according to claim 1, wherein said heating means (1) aremaintained in operation for at least five seconds after starting. 13.Method according to claim 1, wherein said engine torque delivered bysaid engine (M) is evaluated by measuring a position of an acceleratorpedal (10).
 14. Method according to claim 1, wherein said heating means(1) comprise a hot tip whose temperature increases from a temperaturegreater than or equal to 20° C. to a temperature greater than or equalto 1000° C. within six seconds of being started.
 15. Method according toclaim 14, wherein said heating means (1) comprise Mini-Igniter® typeigniters.
 16. Device for implementing a the backwashing method claim 1,said device comprising heating means (1) for heating said filter (F),control means (5) for controlling said heating means, and a computer (7)for managing said control means (5), said device further comprisingdetection means (9, 10) for detecting the delivery by said engine (M) ofan engine torque below a predetermined bottom threshold (C_(bottom)),said computer (7) commanding starting of said heating means (1) inresponse to said detection, said heating means (1) comprising at leastone ceramic igniter adapted to heat by direct contact particlesdeposited on a filter body of said filter (F) and able to reach atemperature of 1000° C. within 3 to 6 seconds.
 17. Device according toclaim 16, wherein hot tips of said heating means (1) are disposed insidesaid filter body or on an outside lateral surface (2) of said filterbody, preferably without projecting from said surface (2).
 18. Deviceaccording to claim 16, including detection means (15) for detectingrotation of said engine (M).
 19. (canceled)
 20. Device for implementingthe backwashing method claim 2, said device comprising heating means (1)for heating said filter (F), control means (5) for controlling saidheating means, and a computer (7) for managing said control means (5),said device further comprising detection means (9, 10) for detecting thedelivery by said engine (M) of an engine torque below a predeterminedbottom threshold (C_(bottom)), said computer (7) commanding starting ofsaid heating means (1) in response to said detection, said heating means(1) comprising at least one ceramic igniter adapted to heat by directcontact particles deposited on a filter body of said filter (F) and ableto reach a temperature of 1000° C. within 3 to 6 seconds.
 21. Device forimplementing the backwashing method claim 3, said device comprisingheating means (1) for heating said filter (F), control means (5) forcontrolling said heating means, and a computer (7) for managing saidcontrol means (5), said device further comprising detection means (9,10) for detecting the delivery by said engine (M) of an engine torquebelow a predetermined bottom threshold (C_(bottom)), said computer (7)commanding starting of said heating means (1) in response to saiddetection, said heating means (1) comprising at least one ceramicigniter adapted to heat by direct contact particles deposited on afilter body of said filter (F) and able to reach a temperature of 1000°C. within 3 to 6 seconds.